Vestibulo-ocular reflexes (VORs) are responsible for maintaining stability of images on the retina during head movements. Many of the behavioral features and neurophysiological mechanisms underlying control of angular VORs have been elucidated. Comparatively little is known about the organization of otolith-ocular reflexes. These reflexes can be divided into three categories according to the type of compensatory eye movements arising from their activation. Ocular counterrolling is the otolith-ocular reflex that causes torsional movement of the eyes around a line of sight in response to dynamic and static lateral head tilt. Translational linear VORs are eye movements that occur in response to dynamic linear movements of the head. The nystagmus induced by constant velocity rotation about an off-vertical axis results from an angular velocity signal generated by central processing of otolith activity. Vestibular-nerve afferents innervating the otoliths cannot distinguish between movements leading to each of these three responses. Research described in this project will define mechanisms involved in specification and adaptive control of otolith-ocular reflexes. Three dimensional and vergence eye movements as well as single-unit activity in the vestibular nuclei will be recorded in alert squirrel monkeys before and after bilateral inactivation of all semicircular canals with a plugging procedure. Hypotheses related to processes that distinguish tilt from translational responses will be tested. The role of angular velocity signals, phasic-tonic responses of irregularly discharging otolith afferents, and frequency-specific filtering of otolith signals in these processes will be determined. The physiological properties of neurons in the vestibular nuclei involved in otolith-ocular control will be defined. Profiles of otolith and semicircular canal afferent inputs to these neurons will be identified in relation to translational and tilt stimuli. Studies of adaptation introduced by optokinetic stimuli in association with motions producing specific profiles of otolith and semicircular canal activation will provide further information about signals used to distinguish these responses. Transfer of adapted VOR gain to reflexes induced by differing combinations of semicircular canal and otolith activity will identify points of convergence in these pathways and define mechanisms involved in generation of angular velocity signals from otolith activity. This research will enhance our understanding of otolith-ocular responses and the processes involved in short-term VOR adaptation. The studies are directly relevant to the diagnosis and treatment of dysfunction in human otolith-ocular reflexes.